1. Field of the Invention
The present invention relates to a turbo decoder and a dynamic decoding method used for it, and in particular, to a method of controlling the number of times of repetition of decoding in the turbo decoder.
2. Description of the Related Art
To prevent deterioration of receiving quality of a radio communications system, a high error-correction coding technology is essential. Turbo coding is an error-correction coding method for realizing transmission characteristics close to a limit of information theory, and is characterized by concatenation encoding for creating a code word by connecting sequences in which an information sequence and the information sequence rearranged are encoded by element encoders respectively and repetition decoding for repeatedly performing decoding corresponding to each element code while utilizing the other decoding result (refer to pp. 5 to 6 and FIG. 2, Japanese Patent Laid-Open No. 2002-190745 for instance).
As for a conventional repetition decoding method, it is ordinary that the number of times of repetition is prescribed in advance. The number of times of repetition is set based on a result of performance evaluation by means of computer simulation and so on.
And no control for dynamically changing the number of times of repetition is normally exerted. In general, soft input and soft output decoding performed a plurality of times in the repetition decoding has a large calculation amount.
As for a conventional repetition decoding method, however, there are the cases where most of receiving sequences are correctly decoded by repeating decoding operation a few times due to an unstable state of a communication channel. And even though the correct decoding results are already obtained by a general decoding method of repetition of a fixed number of times, repetition decoding is implemented a predetermined number of times.
On the other hand, errors have significantly decreased as to the conventional repetition decoding method, and there are the cases where it is very likely to have all the errors corrected by performing a decoding process once more. However, if a conventional turbo decoder performs the decoding a set number of times, it outputs the decoding results including the errors as-is without considering this state. Therefore, a dynamic decoding method is a subject for the sake of realizing an effective turbo decoding.